Raman spectroscopy is a well-known spectroscopic technique for performing chemical analysis. In conventional Raman spectroscopy, high intensity monochromatic light provided by a light source, such as a laser, is directed onto an analyte (or sample) that is to be chemically analyzed. The analyte may contain a single molecular species or mixtures of different molecular species. Furthermore, Raman spectroscopy may be performed on a number of different types of molecular configurations, such as organic and inorganic molecules in either crystalline or amorphous states.
The majority of the incident photons of the light are elastically scattered by the analyte molecule. In other words, the scattered photons have the same frequency and, thus, the same energy (and, therefore, the same frequency, wavelength, or wavenumber), as the photons that were incident on the analyte. However, a small fraction of the photons (i.e., 1 in 107 photons) are inelastically scattered by the analyte molecule. These inelastically scattered photons have a different energy than the incident photons. This inelastic scattering of photons is termed the “Raman effect.” The inelastically scattered photons may have frequencies greater than or, more typically, less than the frequency of the incident photons. When an incident photon collides with a molecule, energy may be transferred from the photon to the molecule or from the molecule to the photon. When energy is transferred from the photon to the molecule, the scattered photon will then emerge from the sample having a lower energy and a corresponding lower frequency. These lower-energy Raman scattered photons are commonly referred to in Raman spectroscopy as the “Stokes radiation.” A small fraction of the analyte molecules may be in an energetically excited state before an incident photon collides with the molecules. When an incident photon collides with an excited molecule, energy may be transferred from the molecule to the photon, which will then emerge from the sample having a higher energy and a corresponding higher frequency than the incident photon. These higher-energy Raman scattered photons are commonly referred to in Raman spectroscopy as the “anti-Stokes radiation.”
The Stokes and the anti-Stokes radiation is detected by a detector, such as a photomultiplier or a wavelength-dispersive spectrometer, which coverts the energy of the impinging photons into an electrical signal. The characteristics of the electrical signal are at least partially a function of the energy (or wavelength, frequency, wave number, etc.) of the impinging photons and the number of the impinging photons (intensity). The electrical signal generated by the detector can be used to produce a spectral graph of intensity as a function of frequency for the detected Raman signal (i.e., the Stokes and anti-Stokes radiation). By plotting the frequency of the inelastically scattered Raman photons against intensity, a unique Raman spectrum is obtained, which corresponds to the particular analyte. This Raman spectrum may be used for many purposes, such as identifying chemical species, identifying chemical states or bonding of atoms and molecules, and determining physical and chemical properties of the analyte.
Since the intensity of the Raman scattered photons is low, intense laser light sources are typically employed to provide the excitation radiation. Surface enhanced Raman spectroscopy (SERS) is another Raman spectroscopy technique that has been developed to increase the Raman signal produced by an analyte and to allow surface studies of an analyte. In SERS, analyte molecules are adsorbed onto or positioned near a specially prepared metal surface. Typically, the metal surface is made from gold, silver, copper, platinum, or aluminum. The intensity of the Raman scattered photons from a molecule adsorbed on such a metal surface is typically about 104–106 greater than conventional Raman Spectroscopy and can be as high as 108–1014. In other words, more photons are inelastically scattered by the analyte molecules in SERS compared to conventional Raman spectroscopy.
The surface enhancement of the Raman signal in SERS is currently attributed to two primary mechanisms: electromagnetic field enhancement and chemical enhancement. The enhancement of the Raman signal is at least partially dependent on the surface roughness or surface features of the metal surface. In SERS, a strong electromagnetic field is present in the areas adjacent to and near the metallic surface when the surface is irradiated by the excitation radiation. This electromagnetic field is experienced by the analyte molecules adjacent to the surface. This strong electromagnetic field enhances the Raman signal emitted from the analyte, which is, at least in part, proportional to the square of the enhanced electromagnetic field. Thus, SERS may be used to perform, for example, surface studies and studies of material monolayers adsorbed on metals. While SERS is an effective chemical analysis tool, it requires rather large and powerful laser light sources. A typical SERS system occupies a large table and is not particularly portable.
Accordingly, there is a need for a more compact and portable SERS system. There is also a need for an excitation radiation source that requires less power during operation that also will enhance, simultaneously, the intensity of the Raman signal to enable more sensitive chemical analysis.